1,720,997 research outputs found
Furanyl Cyclic Ethers: Single and Double Diastereoselectivity in the Synthesis of 2,4-Di and 2,4,5-Trisubstituted Tetrahydropyrans
Full text linkCombining the desymmetrization of a prochiral bis-hydroxymethyl group with the epimerization of a chiral furanyl ether in a single transformation, high levels of double diastereoselectivity have been achieved in a synthesis of 2,4,5-trisubstituted tetrahydropyrans, which proceeds under thermodynamic control
Metal–organic frameworks constructed from Group 1 metals (Li, Na) and silicon-centered linkers
Full text linkA series of “light metal” metal–organic frameworks containing secondary building units (SBUs) based on Li+ and Na+ cations have been prepared using the silicon-centered linkers MexSi(p-C6H4CO2H)4-x (x = 2, 1, 0). The unipositive charge, small size, and oxophilic nature of the metal cations give rise to some unusual and unique SBUs, including a three-dimensional nodal structure built from sodium and oxygen ions when using the triacid linker (x = 1). The same linker with Li+ cations generated a chiral, helical SBU, formed from achiral starting materials. One-dimensional rod SBUs are observed for the diacid (x = 2) and tetra-acid (x = 0) linkers with both Li+ and Na+ cations, where the larger size of Na+ compared to Li+ leads to subtle differences in the constitution of the metal nodes
Four new families of polynuclear Zn-Ln coordination clusters. Synthetic, topological, magnetic and luminescent aspects
Full text linkThe employment of three structurally related Schiff bases H2L1, H2L2 , and H3L3 with zinc and lanthanide salts under various reaction conditions, gave four families of compounds formulated as [ZnII2LnIII2(L1)4(EtOH)6][ClO4]2 (1-3), [ZnII5Ln(OH)(L1)6(H2O)] (4-6), [(ZnII4LnIII2(OH)2(L2)4(OAc)2(NO3)2(DMF)3]) (7-9), [ZnII2LnIII2(L3)2(NO3)2(CO3)2(CH3OH)2] (10-12) with robust and novel topologies. Synthetic aspects are discussed. A comprehensive topological analysis of all reported ZnII/LnIII CCs with a core nuclearity of four and above is presented and identifies that families (4-6) and (7-9) are the first examples of the 2,3,4M6-1 motif in ZnII/LnIII chemistry. Magnetic studies are presented for the DyIII analogues (1, 7 and 10) are presented, 7 demonstrates field-induced slow relaxation of the magnetization. Fluorescence studies are also discussed
Additive effects in the formation of fluorescent zinc metal–organic frameworks with 5-Hydroxyisophthalate
No full textStructure-directing ammonium cations play an important role in the formation and structural properties of a range of four zinc MOFs containing 5-hydroxyisophthalate. Each MOF contains the same [(Ph?O)?(Zn2)?(OOC?Ph)] secondary building unit to produce different structures dependent on the structure-directing agents added to the synthetic mixture.<br/
Solvent-free synthesis and key intermediate isolation in Ni2Dy2 catalyst development in the domino ring-opening electrocyclization reaction of furfural and amines
No full textA solvent-free methodology that yields trans-4,5-diaminocyclopent-2-enones, main domains of natural products and variety of N-heterocycles, is described. The bimetallic catalyst [NiII2DyIII2L4(DMF)6] 2(OTf) 2(DMF) (1) promotes the domino reaction of furfural and amines, with loadings as low as 0.01%, under stirring or microwave assisted conditions to afford the corresponding frameworks in very good to excellent yields. Crystallographic and theoretical studies shed light on the exclusive formation of the trans-diastereoisomers via a 4π-conrotatory electrocyclization process elucidating the key step in the catalytic process
Modulation of σ-Alkane interactions in [Rh(L2)(alkane)]+ solid-state organometallic (SMOM) systems by variation of the chelating phosphine and alkane: Access to η2,η2-σ-alkane Rh(I), η1-σ-alkane Rh(III) complexes, and alkane encapsulation
Full text linkSolid/gas single-crystal to single-crystal (SC–SC) hydrogenation of appropriate diene precursors forms the corresponding σ-alkane complexes [Rh(Cy2P(CH2)nPCy2)(L)][BArF4] (n = 3, 4) and [tiebar above startRhH(Cy2P(CH2)2(tiebar above endCH)(CH2)2PCy2)(L)][BArF4] (n = 5, L = norbornane, NBA; cyclooctane, COA). Their structures, as determined by single-crystal X-ray diffraction, have cations exhibiting Rh···H–C σ-interactions which are modulated by both the chelating ligand and the identity of the alkane, while all sit in an octahedral anion microenvironment. These range from chelating η2,η2 Rh···H–C (e.g., [Rh(Cy2P(CH2)nPCy2)(η2η2-NBA)][BArF4], n = 3 and 4), through to more weakly bound η1 Rh···H–C in which C–H activation of the chelate backbone has also occurred (e.g., [tiebar above startRhH(Cy2P(CH2)2(tiebar above endCH)(CH2)2PCy2)(η1-COA)][BArF4]) and ultimately to systems where the alkane is not ligated with the metal center, but sits encapsulated in the supporting anion microenvironment, [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF4], in which the metal center instead forms two intramolecular agostic η1 Rh···H–C interactions with the phosphine cyclohexyl groups. CH2Cl2 adducts formed by displacement of the η1-alkanes in solution (n = 5; L = NBA, COA), [tiebar above startRhH(Cy2P(CH2)2(tiebar above endCH)(CH2)2PCy2)(κ1-ClCH2Cl)][BArF4], are characterized crystallographically. Analyses via periodic DFT, QTAIM, NBO, and NCI calculations, alongside variable temperature solid-state NMR spectroscopy, provide snapshots marking the onset of Rh···alkane interactions along a C–H activation trajectory. These are negligible in [Rh(Cy2P(CH2)3PCy2)][COA⊂BArF4]; in [tiebar above startRhH(Cy2P(CH2)2(tiebar above endCH)(CH2)2PCy2)(η1-COA)][BArF4], σC–H → Rh σ-donation is supported by Rh → σ*C–H “pregostic” donation, and in [Rh(Cy2P(CH2)nPCy2)(η2η2-NBA)][BArF4] (n = 2–4), σ-donation dominates, supported by classical Rh(dπ) → σ*C–H π-back-donation. Dispersive interactions with the [BArF4]− anions and Cy substituents further stabilize the alkanes within the binding pocket
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
